Fine Aerosol Acidity and Water during Summer in the Eastern North Atlantic

• Published in: Atmosphere
• Main Authors: Nah, Theodora, Yang, Junwei, Wang, Jian, Sullivan, Amy P., Weber, Rodney J.
• Language: unknown
• Published: 2021
• Subjects: 54 ENVIRONMENTAL SCIENCES; North Atlantic
• Online Access: http://www.osti.gov/servlets/purl/1813546; https://www.osti.gov/biblio/1813546; https://doi.org/10.3390/atmos12081040

Aerosol pH governs many important atmospheric processes that occur in the marine boundary layer, including regulating halogen and sulfur chemistries, and nutrient fertilization of surface ocean waters. In this study, we investigated the acidity of PM 1 over the eastern North Atlantic during the Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) aircraft campaign. The ISORROPIA-II thermodynamic model was used to predict PM 1 pH and water. We first investigated the sensitivities of PM 1 pH and water predictions to gas-phase NH 3 and HNO 3 concentrations. Our sensitivity analysis indicated that even though NH 3 and HNO 3 were present at very low concentrations in the eastern North Atlantic during the campaign, PM 1 pH calculations can still be sensitive to NH 3 concentrations. Specifically, NH 3 was needed to constrain the pH of populations of PM 1 that had low mass concentrations of NH 4 + and non-volatile cations (NVCs). We next assumed that gas-phase NH 3 and HNO 3 concentrations during the campaign were 0.15 and 0.09 µg m -3 , respectively, based on previous measurements conducted in the eastern North Atlantic. Using the assumption that PM 1 were internally mixed (i.e., bulk PM 1 ), we determined that PM 1 pH ranged from 0.3–8.6, with a mean pH of 5.0 ± 2.3. The pH depended on both $H^{+}_{air}$ and $W_{i}$. $H^{+}_{air}$ was controlled primarily by the NVCs/SO42- molar ratio, while Wi was controlled by the SO42 mass concentration and RH. Changes in pH with altitude were driven primarily by changes in SO 4 2- . Since aerosols in marine atmospheres are rarely internally mixed, the scenario where non-sea salt species and sea-salt species were present in two separate aerosol modes in the PM 1 (i.e., completely externally mixed) was also considered. Smaller pH values were predicted for the aerosol mode comprised only of non-sea salt species compared to the bulk PM 1 (difference of around 1 unit on average). This was due to the exclusion of sea-salt species (especially hygroscopic alkaline NVCs) in ...